1. Field of the Invention
The present invention relates to multilayer blow molded containers having excellent gas-barrier properties, good interlayer adhesion, and excellent thermal stability. The present invention also relates to methods for preparing such multilayer blow molded containers.
2. Discussion of the Background
Since ethylene-vinyl alcohol copolymers (hereinafter, referred to as EVOH) are excellent in transparency, barrier properties against oxygen, carbon dioxide and nitrogen and oil resistance, and also excellent in melt-molding properties, they are used as wrapping materials for foods, medicaments and the like while making the best use of these properties. In recent years, in order to make up for gas-barrier properties at higher humidity, impact resistance and the like, EVOH are frequently used while being laminated with other thermoplastic resins excellent in moisture-proof properties and impact resistance. Also, in the field of hollow containers, a structure or constitution that contains a resin such as polyethylene, polypropylene, polyethylene terephthalate (PET), etc. in inner and outer layers and EVOH in an intermediate layer is favored.
Containers of thermoplastic polyesters, typically PET, are used in many fields, since they are excellent in various properties including transparency, mechanical properties, flavor-barrier properties and the like, low in possibility of elution of residual monomers and harmful additives upon molding, and excellent in hygienic quality and safety. On the other hand, since polypropylene is inactive to blood, has a low thermal shrinkage, and is low in price, polypropylene containers are attracting attention for use with medicaments and foods when the contents are filled while hot, or so-called hot-filled.
Concerning the polypropylene containers, Japanese Patent Laid Open No. 139228/1980 discloses a technique for improving adhesion strength between a polypropylene layer and an EVOH layer by adding an adhesive resin made of a maleic anhydride-modified polypropylene or the like to the polypropylene layer. It has been known that, in this case, an alkali metal such as sodium, etc. contained in the EVOH is effective for obtaining sufficient adhesion strength because it exhibits a catalytic effect.